(1) Field of the Invention
The present invention relates to image forming apparatuses such as printers and copiers, and particularly relates to an art of controlling misalignment in image forming start position between colors that occurs in image forming apparatuses for forming a color image by layering toner images of the colors.
(2) Description of the Related Art
In recent years, image forming apparatuses employing an electronic photography system use a semiconductor laser as a light source for writing onto an image carrying surface of an image carrier such as a photoreceptor. The image forming apparatuses perform exposure-scanning on the image carrying surface by laser light emitted from the semiconductor laser to form an electrostatic latent image on the image carrying surface.
Exposure-scanning is performed by turning on and off emission from the semiconductor laser based on image data. Specifically, on-off control of emission from the semiconductor laser is performed by inputting an image signal that is a pulse signal. The pulse signal is generated based on the image data, and instructs to turn on and off emission and an emission duration from the semiconductor laser.
When the image signal instructs emission from the semiconductor laser (turn-on of emission), driving current is supplied to the semiconductor laser from a laser driving device for a duration corresponding to a pulse width of the image signal.
FIG. 21 is a timing chart showing a correspondence relationship among an image signal input to a laser driving device, driving current supplied to a semiconductor laser in accordance with the input image signal, and output of laser light emitted from the semiconductor laser in accordance with the supplied driving current. In FIG. 21, section (a) indicates the image signal input to the laser driving device, section (b) indicates the driving current supplied to the semiconductor laser in accordance with the input image signal, and section (c) indicates output of the laser light emitted from the semiconductor laser in accordance with the supplied driving current.
Here, the image signal in the low status instructs turn-on of emission from the semiconductor laser. The image signal in the high status instructs turn-off of emission from the semiconductor laser. The image signal instructs an emission duration of the semiconductor laser depending on a pulse width thereof. Also, while the image signal is in the low status, the driving current is supplied to the semiconductor laser. While the image signal is in the high status, the driving current is not supplied to the semiconductor laser. Furthermore, while the image signal is in the low status, laser light is output. While the image signal is in the high status, laser light is not output.
As shown in the figure, supply of the driving current to the semiconductor laser is started in accordance with input of the image signal instructing emission from the semiconductor laser to the laser driving device. However, output of laser light is delayed behind input of the image signal instructing emission.
This is because after the image signal instructing emission from the semiconductor laser is input and supply of the driving current to the semiconductor laser is started, a predetermined duration is necessary for the semiconductor laser to generate a carrier having a concentration at which laser oscillation is possible.
As a result, an actual emission duration of laser light is shorter than the emission duration from the semiconductor laser which is instructed by the image signal by a duration corresponding to output delay of the laser light. In response to this problem, for example, Patent Literature 1 (Japanese Patent Application Publication No. 2011-167898) discloses an art of expanding a pulse width of an image signal (equivalent to a light emitting signal in Patent Literature 1) to reserve an emission duration corresponding to the image signal.
According to this art, the pulse width of the image signal is expanded by a pulse width corresponding to the output delay, and as a result the emission duration is extended. Therefore, it is possible to adjust the emission duration of the laser light so as to correspond to the image signal.
However, the above art cannot adjust start delay of emission of laser light that is caused by an emission delay duration from when the image signal instructs emission from the semiconductor laser to when the semiconductor laser emits laser light. Also, as disclosed in Patent Literature 2 (Japanese Patent Application Publication No. 2011-235578), the emission delay duration varies depending on an amount of laser light emitted from the semiconductor laser (exposure intensity) and so on (the emission delay duration decreases as the amount of laser light increases).
For this reason, in the case where image forming apparatuses, which perform image formation by layering images of four colors of yellow, magenta, cyan, and black, such as full-color image forming apparatuses, use a different exposure intensity of a semiconductor laser for each color, an emission delay duration also differs for each color. This causes misalignment in writing start position in a scanning direction for exposure-scanning between the colors, and as a result causes color misregistration due to the misalignment in image forming start position in the scanning direction between the colors.